Cascadia's Fault - Jerry Thompson [46]
The tip-off, according to Ando and Balazs, should occur whenever we see the outer coast of the Pacific Northwest start to dip down and get pulled under by the Juan de Fuca plate. Not surprisingly, they recommended constant vigilance by a team of surveyors with state-of-the-art equipment to spot any change along the beach. In the meantime, because the Coast Mountains are now tilting eastward instead of toward the sea, they assured us that a large thrust earthquake from Cascadia’s fault is “not expected in the near future.”
Then along came the eruption of Mount St. Helens one year later. How could a violent explosion like this not be the sign that convinces all and sundry that Cascadia is still very active and that a tectonic disaster is looming? I suppose the first and simplest explanation was that the debate about Cascadia as “a special case” was happening mostly within the confines of the science community. The general public was not reading these new technical papers, not attending the scientific meetings, and therefore they did not know, for the most part, that Cascadia’s fault even existed.
But why did so many scientists who had read the new literature still hesitate?
Gary Carver, who was still mapping thrust faults in the rumpled hills around the Humboldt Bay nuclear plant at the time Mount St. Helens blew, knew that the majority of scientists were skeptical of the Cascadia disaster scenario. Why would the volcanic eruption not have been seen as proof positive of active subduction? He told me that an eruption could still happen even after subduction had stopped.
Presumably, if the Juan de Fuca plate stopped moving tomorrow, the segment of the down-going slab that had already been pulled toward the earth’s hot interior would have begun to melt. Plumes of magma would already be rising up beneath the arc of volcanic mountains. So there could be a lag of who knows how many years—hundreds, maybe thousands—between the end of Cascadia’s plate motion and the final eruption of Mount St. Helens or one of its neighbors. From that perspective, the St. Helens blast didn’t prove anything.
Apparently what everyone needed and wanted was forensic evidence that there had been specific Cascadia earthquakes at specific times in the past. Not just hypothetical scenarios, not just signs that the beaches had been hoisted or the mountains tilted, or even that there had been smaller fractures in the continental crust near the California coast. The only thing that could finally put an end to all the back and forth would be tangible signs of past ruptures along the entire subduction zone. And once again, a clue about where that proof might be found was layered in the story written by Walter Sullivan of the New York Times, just before Mount St. Helens blew.
The so-called missing trench had to be significant somehow. There were trenches at all the other converging plate boundaries, so why would Cascadia’s subduction zone be different? From what I’d read in the science journals, there was a bit of a trench off the west coast, although it was shallow compared to the others and filled with sediment. Why was the down-going angle of the Juan de Fuca plate almost horizontal while the others were steeper and deeper? And what did it matter that the crack was full of mud? The never-ending dump of sand and silt from the turbulent Columbia River and many others along the Pacific Northwest coast had all but buried the fault, so as with the Juan de Fuca Ridge, nobody knew the trench was there at first.
Trenches at other plate boundaries in deeper parts of the Pacific are located far enough away from the outflow of big mountain rivers that silt and sediment can’t hide the evidence